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C:UsersMikeDocumentsLifetime ReliabilityLRS Maintenance MethodologyMaintenance_Job_Priority_Rating_Tips.docx 1
Masters of reliability and maintenance excellence
Tips on Maintenance Job Priority Rating
Abstract:
Tips on Maintenance Job Priority Rating: To help select which work orders to do first in situations
of resource shortage many CMMS provide calculations for maintenance work order priority.
Deciding maintenance work priority is a risk decision. The presence of risk totally changes the way
to allocate maintenance job priority if you want to compare situations equally1
. When you work
with risk you cannot use a linear priority scale. Using linear priority ranking gives the wrong order
of importance for doing maintenance work.
Keywords: maintenance work priority, work order priority ranking, job priority matrix
NOTE: You can now do our online 10 module Maintenance Planning and Scheduling training course with
certificate once completed. Read more at Online Maintenance Planning and Scheduling Training Course.
When you have to decide between maintenance jobs how do you chose which to do ahead of others?
Many Computerised Maintenance Management Systems (CMMS) let you use asset priority and/or
work priority to help schedule maintenance work orders. Selecting when to do maintenance work,
where the consequence of being wrong is an operational failure, perhaps even death in severe
circumstances, involves a risk scenario. Deciding which maintenance work to do is a risk based
decision, as the choice may lead to the failure of equipment that would not have failed but for the
decision to do the work at a particular time and not some other. Maintenance priority depends on
the size of the risk of being wrong in waiting since the wrong choice will cost the business fortunes.
In prioritising maintenance work you balance two factors – the total business-wide consequences of
failure, and when to do the work (Whether the work is effective in preventing failure is a separate
issue.). It seems sensible that as the consequences of failure worsen it becomes more important to
make sure that a failure does not happen. If you continue this thinking you would prioritise and
focus on doing high consequence work first and less important work later. This is the ‘gut feel’
approach we all instinctively use. For setting maintenance priorities you need a measure of the risk.
The standard risk equation is: Risk ($/yr) = Likelihood (/yr) x Consequence ($) Eq. 1
As a log-log equation it is: Log Risk = Log Likelihood + Log Consequence Eq. 2
An event with a consequence of $10,000 every time it happens that occurs 10 times a year will cost
the organisation $100,000 per year. Another event that costs $100,000 and happens once a year
costs the organisation $100,000 per year. These two situations are of equal risk but our perceptions
of them are vastly different. We would do everything possible to stop a $100,000 single event and
do little to stop a $10,000 event. Yet the organisation loses just as much money over a year from
both. If each were to be ‘gut feel’ prioritised the single $100,000 event would get a higher priority
than the $10,000 event.
If you have two maintenance work orders to do; one a preventive maintenance job (PM) that
prevents a $10,000 event from occurring ten times a year, and one to fix a once-a-year $100,000
breakdown after it happened, which one is the more important to do? Anyone using a linear priority
scale from 1 to 5, with 5 being the highest priority, would likely give 5 to repair the $100,000
breakdown and a 1 to the $10,000 preventive maintenance job. They may even cancel the PM and
divert the manpower and effort to the $100,000 urgent work. The priority scale seems to justify
doing the breakdown repair ahead of preventive work as a sensible thing to do.
1
Thanks to Howard Witt of Witt Consulting, Sydney, Australia for his advice and questioning of my logic.

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But each scenario involves business risk and must be ranked by its risk priority and not a linear
priority. Risk ranking is vastly different to work prioritised by linear scale. With risk based priority
ranking you develop a risk table using a log10-log10 scale. Equation 2 is the risk equation as a log10
formula. To find the log10 value for the risk we add together the log10 numbers for Likelihood and
Consequence. The log10 of 10 is 1 (101
), of 100 it is 2 (102
), of 1,000 it is 3 (103
), and so on.
Notice that the log10 value is the same number as the exponent, which allows us to simplify notation
to 1, 2, 3, etc. As long as we always use log10 we know that each number is ten times different to its
neighbours. The values in the cells of Table 1 are calculated with Equation 1 and are the annual
cost of carrying a level of risk. In Table 2, consequence and likelihood changes ten times in value
with each number, so that 2 is ten times 1, 3 is ten times 2, 5 is ten thousand times 1, and so on2
.
Consequence (Cost per Event)
Likelihood (Frequency) Negligible Minor Important Major Severe Disaster
Likelihood
of Event
Event Count
per Year
$10 $100 $1,000 $10K $100K $1,000K
Certain 10 $100
/yr
$1K
/yr
$10K
/yr
$100K
/yr
$1,000K
/yr
$10,000K
/yr
Almost
Certain
1 $10
/yr
$100
/yr
$1K
/yr
$10K
/yr
$100K
/yr
$1,000K
/yr
Possible 0.1
(once in 10 yrs)
$1
/yr
$10
/yr
$100
/yr
$1K
/yr
$10K
/yr
$100K
/yr
Rare 0.01
(once in 100 yrs)
$0.10
/yr
$1
/yr
$10
/yr
$100
/yr
$1K
/yr
$10K
/yr
Very Rare 0.001
(once in 1000 yrs)
$0.01
/yr
$0.10
/yr
$1
/yr
$10
/yr
$100
/yr
$1K
/yr
Table 1 – Financial Value of Risk
The cell values in Table 2 sum log10 likelihood and log10 consequence and also represent the risk.
They correspond to the scale of impact. Though not the actual log10 value from using Equation 2
they are still signify multiples of ten. The table ranks business risk importance. The same numbers
in cells represent the same amount of risk. The colours represent the levels of risk. This approach
is standard risk management methodology and commonly used in industry to determine
occupational health and safety risk.
Risk Level Consequences
Red = Extreme Safety: Describe Negligible through to Disaster events
Amber = High Environmental: Describe Negligible through to Disaster events
Yellow = Medium Reputation: Describe Negligible through to Disaster events
Green = Low Financial: Describe Negligible through to Disaster costs
Blue = Slight Other: Describe Negligible through to Disaster events
Likelihood of
Event
Event Count
per Year
Negligible Minor Important Major Severe Disaster
1 2 3 4 5 6
Certain 10 5 6 7 8 9 10 11
Almost
Certain
1 4 5 6 7 8 9 10
Possible 0.1
(once in 10 yrs)
3 4 5 6 7 8 9
Rare 0.01
(once in 100 yrs)
2 3 4 5 6 7 8
Very Rare 0.001
(once in 1000 yrs)
1 2 3 4 5 6 7
Table 2 - Risk Value of an Event
2
AS4360:2004 Australian Risk Management Standard

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When the method is used for maintenance work priority the consequences remain the same but we
need to find words that represent the likelihood of failure if the work is not done. Maintenance
work can be broadly grouped into PM work done to a schedule based on usage and/or time, and
work requested that reflects changing fortunes and problems in an operation.
Table 3 shows an attempt to use words to describe the likelihood impact of maintenance work. If
the item is already failed it is a breakdown. But it does not mean you do a breakdown job if there is
higher priority work. A risk table makes it clear which maintenance work is more important for the
business and so you are less inclined to simply respond to the most insistent person, or to
misunderstand what risk a job really is and where you ought to put your priorities. It is vital to
remember that the risk scale is in multiples of ten. Each number is ten times the impact of its
neighbours. A maintenance job with a risk priority of 8 is not twice the importance of one given a
priority of 4, which is what a linear scale implies (on a linear scale 4 is half of 8), it is 10,000 times
more risky (where 10,000 x 10,000 = 10,000,000 i.e. 104
x 104
= 108
and as log10 4 + 4 = 8).
Understanding work risk priority ranking is vital if you want to do maintenance that paybacks the
greatest value to your business.
Risk Level Consequences
Red = Extreme Safety: Describe Negligible through to Disaster events
Amber = High Environmental: Describe Negligible through to Disaster events
Yellow = Medium Reputation: Describe Negligible through to Disaster events
Green = Low Financial: Describe Negligible through to Disaster costs
Blue = Slight Other: Describe Negligible through to Disaster events
Impact Of Work Order Delay Negligible Minor Important Major Severe Disaster
PM Work Requested Work 1 2 3 4 5 6
Certain to have negative
consequences if not
done by scheduled date
Item has failed or
failure is imminent 5 6 7 8 9 10 11
Almost certain to have
negative consequences if
done later than schedule
No adverse
consequences if
done within 1 – 3
days of request date
4 5 6 7 8 9 10
Possible to have
rescheduled to next
scheduled date
No adverse
consequences if
done within 10 – 14
days
3 4 5 6 7 8 9
Rare to have negative
consequences if
rescheduled to next
scheduled date
No adverse
consequences if
done within 2 – 3
months
2 3 4 5 6 7 8
Very rare to have
rescheduled several
times
No adverse
consequences if
done within 2 – 3
years
1 2 3 4 5 6 7
Table 3 - Log-Log of Maintenance Work Risk
You can even go a step further and advise people how you want them to behave in response to the
risk by including the response to take, as shown in Table 4. Going back to the two maintenance
work order scenarios—a $100,000 breakdown and a PM to stop a $10,000 failure—we can now rate
them using the risk ranking for work priority. The breakdown consequence is major and the
likelihood is a certain failure, which gives a priority of 9. The PM job prevents a failure ten times a
year, which would be a certain $100,000 a year lost—the PM job is a 9 as well. Both jobs are equal
in priority to the business and both need to be done urgently.
A linear priority would have given the breakdown a maximum value and the PM job could very
likely have been cancelled. But once the work is treated as a business risk the linear scale proves to
be nonsense. This is the trap when using a linear priority scale for scheduling maintenance—the
wrong jobs get done. Scheduling maintenance work is not the same situation as scheduling a list of
tasks in a diary—it is not time management. Where in a diary we can list tasks by numeric order of
C
B
A

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importance and do them in that order, we cannot do so with maintenance because we are dealing
with risk, and risk must be treated as orders of magnitude and not linear numbers.
Risk Level Response Consequences
Red = Extreme
Immediately allocate all
necessary resources,
prioritise work and do the
job
Safety: Describe Negligible through to Disaster events
Amber = High
Immediately plan and
prepare, then do the job Environmental: Describe Negligible through to Disaster events
Yellow = Medium
With priority, plan the
work and then schedule
the job
Reputation: Describe Negligible through to Disaster events
Green = Low
In a timely manner, plan
the work and schedule
the job
Financial: Describe Negligible through to Disaster costs
Blue = Slight
Gather work together and
do as a scheduled
campaign
Other: Describe Negligible through to Disaster events
Impact Of Work Order Delay Negligible Minor Important Major Severe Disaster
PM Work
Requested
work
1 2 3 4 5 6
Certain to have negative
consequences if
rescheduled to next
scheduled date
Item has failed or
failure is imminent 5 6 7 8 9 10 11
Almost certain to have
rescheduled to next
scheduled date
No consequences
if done within 1-3
days of request
date
4 5 6 7 8 9 10
Possible to have negative
consequences if
rescheduled to next
scheduled date
No consequences
if done within 10 –
14 days
3 4 5 6 7 8 9
Rare to have negative
consequences if
rescheduled to next
scheduled date
No consequences
if done within 2 - 3
months
2 3 4 5 6 7 8
Very rare to have negative
consequences if
rescheduled to next
scheduled date
No consequences
if done within 2 - 3
years
1 2 3 4 5 6 7
Table 4 - Maintenance Work Order Priority Rank
Often people use a simpler scale of A-B-C to represent respectively High-Moderate-Low risk. An
ABC risk scale is overlayed on the risk matrix in Table 3. It is simple to use, but it is too simple to
be of use for setting maintenance priorities of work orders. Only ‘A’ and ‘B’ are likely to get
scheduled (and even those will be in the wrong order due to the linear nature of using an ABC
scale). The ‘C’ work orders wait for resources until often the equipment fails and the job becomes
an ‘A’ priority (and of course then it gets done). The other deception in ABC work priority ranking
is that in reality ‘A’ level risks do not actually happen often, yet there will be a disproportionate
number of work orders ranked ‘A’. It causes limited resources to be used ineffectively, with jobs
being done earlier than they should have been. With the cell numbering of Table 4 there are 10
priority levels to quickly differentiate the importance of a maintenance job by orders of magnitude.
You do the highest numbers first because that is where the really big money is for the business. All
cells with the same value carry equal risk and apart from convenience, it does not matter in which
order you do work of the same business risk rating.
Some risk professionals tell you not to assume the worst possible thing that could happen, rather to
assume what could reasonably be expected to go bad in the circumstances i.e. a pessimistic
assumption but not the absolutely worst credible. I take the very worst possible, as that is why
calamities like Flixborough in UK, Bhopal in India, BP Texas Refinery Explosion, sinking of the
Titanic, Longford Gas Plant explosion in Australia, Piper Alpha in the North Sea, the Challenger
Space Shuttle disaster and far too many others, happened; someone said they were not creditable
events and did nothing about it. Catastrophic risk doesn’t work like that—massive risks arise more

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often than by chance unless you prevent them3
. What you want to do is to encourage people to be
proactive and look for things to go wrong. Using a risk matrix to rate maintenance work helps
people see wasted profits and possible disasters and justify action to stop them from happening.
Prioritising maintenance work with a matrix like Table 4 highlights the great importance of doing
scheduled PM work. Many times scheduled work will be delayed when resources are not available
because of apparently higher priority work (Often PMs are mistakenly cancelled to wait for the next
time they come due.). But this is crazy because scheduled work is there to prevent a failure. If a
scheduled predictive maintenance (PdM) job, or condition monitoring (CM), or a preventive
maintenance (PM) job is not done when due, you increase the likelihood of breakdown. Delay
doing those jobs long enough and you guarantee failure. Doing PM and CM work is the first
principle of maintenance management because you proactively keep your equipment healthy.
The priority table warns us about one more important maintenance management principle—a
maintenance group cannot do both urgent work and important work at the same time. In Table 4
the urgent work is shown separate to important work that is not urgent. The group responsible for
urgent work focuses on getting good at reactive maintenance done to high reliability standards. The
group focused on important work gets good at doing high quality work to create high reliability (so
that there will be no urgent work in future). Each group needs a different mentality that cannot be
shared within one group of people—reactive work will always win and kill reliability growth work.
Things Not to Do in Maintenance Work Priority Rating
Asset Priority and Asset Criticality are not necessarily the same meaning. Asset Criticality is the
risk value calculated from the risk equation. Asset Priority is the order of importance of the asset to
the business. It can be a risk value or some other scale, be it numeric like 1, 2, 3. 4, 5, or descriptive
such as low, medium, high, extreme. Similarly, Job Priority and Job Criticality are not necessarily
the same meaning. Job Criticality also is the risk value from the risk equation (we used it above to
set Work Priority), whereas Job Priority is a numeric or descriptive order.
Figure 1 - Wrong Priority by Adding Two Risks
In Figure 1 the values for Asset Criticality and Job Criticality rating come from their respective risk
matrices and are mistakenly used as the axes in a second matrix for selecting the work order priority.
This arrangement adds two risk values together but the final value does not represent the true risk in
the situation it is meant to represent. Each cell is not ten times the value of the one above it or to
the right of it. The approach produces a priority order but it does not correctly reflect the real risk.
The work priority equations used in some CMMS can lead to skewed priority. In Figure 2 the
priority equation is 2 x Job Priority + Asset Priority. By using the word ‘priority’ it causes
confusion as to whether to use linear or risk scales.
3
Ball, P., ‘Critical Mass – how one thing leads to another’, Arrow Books, 2005
Incorrect Method

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If risk scales are used then, like Figure 1, it adds two risks together to create a scale not related to
the risk. Furthermore, doubling the job priority value causes the situational circumstance of a job to
have more value than the importance of an asset. You see the skewed effect because values
diagonally down to the right in a standard risk matrix are now far to the right. It makes getting the
job done more important than what is best for the business. If the axes were linear then the resultant
priority is also linear and does not reflect risk.
Figure 2 – Wrong Equation Skews Priority
Another trap is shown in Figure 3, where the intent was to scale Maintenance Work Order Priority
downward from a highest value of 100. This was done by weighting the values to make them fit the
required scale. The problem is that you can’t scale risk values as you wish and think the result is a
reflection of what the risk actually is. You may achieve the aim of getting a particular scale but the
numbers do not reflect real risk. Work prioritised by this scale will not have the necessary
importance to people. A job with a priority of 20 is not five times less risk of one rated 100. In a
correct risk matrix the difference between a job priority of 10 (5 + 5) and a priority of 2 (1 + 1) is
100 million times more risky (100 x 100,000,000 = 10,000,000,000, or as log10 2 + 8 = 10). This
100-point scale confuses people into thinking that a job with a 70 value is not much more important
than one with a 50 value and so it can wait to be done because the numbers are not that different. In
a normal log10 risk table there would be 100 times the difference in risk.
Figure 3 - Scaling Causes Wrong Priority Connotation
Conclusion
When you decide to prioritise your maintenance work orders you are taking a risk decision. It is a
choice that has great business consequence. Scheduling maintenance work requires understanding
that a small failure can lead to a disaster and the risk is not linear. To protect from gross scheduling
errors make maintenance decisions on a risk based priority matrix, and ensure that your CMMS
uses a real risk calculation and not a convenient way to get a nice scale that misleads your selection.
Mike Sondalini
Incorrect Method
Incorrect Method

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